Drive mechanisms for windshield wipers are provided with switches taking care of further supplying current to the motor of the drive mechanism, after the main switch has been opened by the driver, until the windshield wiper will again have reached its home position also referred to as park position. Correspondingly, the name of this switch is park position switch.
Usually, the park position switches are realised by a switch disk having a plurality of concentric contact paths which are sectionally interrupted and where, in each of the paths, a point-contact element is sliding, establishing an electrically conductive contact towards the contact path. Often, these contact paths are arranged fast on the output gear of the output shaft while the point contacts are formed on contact springs which are formed fast with the housing.
In this arrangement, where one element of the park position switch is connected fast with the output shaft, there are some problems.
Outside influences, such as snow on the windshield, may prevent the windshield wiper blade from reaching its park position. As, however, the output shaft will continue to be driven by the motor until it will have adopted an angular position which corresponds to the park position of the wiper blade, with the movement being unhindered, the wiper arm and wiper blade will be bent elastically as they support themselves on the obstacle. As soon as the motor automatically turns off, the relaxing wiper will cause the output shaft to be rotated back. In consequence, the park position switch will be closed again and current will be supplied to the motor. As a result, the wiper will again be moved against the obstacle and there will again be forming a mechanic stress. This operation will repeat itself until the obstacle has been removed. It is easy to see that, in this action, the gear unit is acted upon by considerable stresses reducing its service life.
Therefore, DE 36 06 891 A1 already suggested to connect one part of the park position switch to the output shaft via a coupling element instead of connecting it fast with the output shaft. In the Published Patent Application mentioned, the contact paths are arranged fast in the housing and the point-contact elements are rotatably supported and connected with the output shaft via a coupling device. However, this principle is also applicable if the point-contact elements are formed fast with the housing and the contact paths are formed on a switch disk rotatably supported in the housing. In this case, the switch disk is connected with the output shaft via a coupling element.
The coupling is of such a design that, in normal operation, when the wiper is driven to perform a pendulum movement, the output shaft will carry along the switch disk so that the park position switch works like a park position switch which is connected fast with the output shaft.
However, if there comes up the above-discussed case where the output shaft is reset, contrary to its actual direction of rotation, the switch disk will not be carried along so that the park position switch, which otherwise, again, would supply current to the motor, will not be closed.
In the Published Patent Application mentioned, the switch device features a design where the rotatable element is rotatably supported on a pivot connected with the housing lid, with the output shaft being axially supported on the front face of the pivot. The disadvantage of this arrangement is that, as compared with a conventional design, the axial extension of the gear housing is increased by at least the height of the housing pivot.
Therefore, in order lower an axial dimension of the housing, this invention suggests to support the rotatably supported switch element on a supporting sleeve formed on the housing lid and to have the output shaft extend into the sleeve.
This arrangement, moreover, is beneficial in that the output shaft can support itself on the housing lid directly. Belonging to the output shaft is also that part of the output gear which extends like a cap over the front face of the output shaft.
The housing lid consists of a base plate of metal, with the supporting sleeve being of plastic injection-moulded onto the metallic base plate. The part of the base plate whereon the output shaft is supported is kept free of plastic so that the axial forces will directly be introduced into the base plate which is directly fastened on the edge of the gear housing.
Provided on the outside of the base plate is a plastic layer wherein conductive tracks are extending via which the point contacts of the park position switch as well as the connected electric motor are supplied with current, and it is an obvious development to design the supporting sleeve and this plastic layer in one piece.
The rotatable switch element consists of an annular disk of plastic the inside edge of which rests on the outside surface of the supporting sleeve. Either the outside of the supporting sleeve or the inside contour of the annular disk may feature a polygonal design so that the supporting sleeve and the annular disk will touch each other only at certain points. Lubricant may be deposited in the clearances between these certain points of contact. The concentric contact paths are formed on the side of the annular disk which faces the lid.
The contact points are formed on contact springs approximately distributed evenly over the circumference of the contact disk so that the pressure applied by the contact springs to the annular disk will cause no tilting moment.
The contact springs are fastened to the bottom side of the plastic layer. To this end, the metallic base plate features appropriate oblong apertures whereby sections of the plastic layer are bared downwards. The contact springs are fastened to these bare spots and connected with the conductive tracks in the plastic layer.